I recently attempted to better understand script execution flow. As Zeek’s scripting language is heavily driven by events, debugging scripts can, at times, be frustrating and surprising. In an effort to reduce both frustration and surprises for others in the community, I wrote this blog.

There are many subtle differences between function, hook, and event types. This post hopefully serves as a way to quickly understand the similarities and differences of these executable types.


While Zeek does support the concept of anonymous, also called lambda, functions, hooks and events cannot be anonymous. They are referenced by their names. As an example, reducer functions in the SumStats framework are often implemented as lambda functions.

Multiple bodies and priorities

Functions cannot have multiple bodies, however, hooks and events can. This means that different scripts can add additional bodies to a hook or event associated with a unique name. When an event or hook is executed, Zeek needs a way to order the execution. This is accomplished with priorities. By default, a hook’s or event’s body has a priority of zero but valid values can be from -10 to 10.

Immediate invocation

Functions and hook bodies are executed immediately. That means if a script is being interpreted and a line contains a function call, execution flow is immediately passed to that function (or hook). This does not happen for events. Events are pushed onto an event queue within Zeek and are handled as time passes.


Functions and hooks cannot be scheduled like events can. Scheduling places an event onto the event queue and is the equivalent to immediately invoking a function or hook. Attempting to schedule a function or a hook results in the same syntax error: “function invoked as an event”.

Default arguments

Functions, hooks, and events all support default arguments.

Container argument mutability

When argument types are container types, such as records, mutating the arguments within the body (of a function, hook, or event) causes the record to retain that mutation. Essentially, container types are passed by reference while atomic types are passed by value.

Alternate declarations

Hooks and events do support alternate prototype declarations. This means that a set or scripts may define a single event (or hook) name multiple times with different argument sets. This is often referred to as overloading in other languages. Functions do not support alternate prototype declarations.

Return values

All functions must return a value. However, functions with no explicit return type implicitly return void. This can seem a bit odd as void isn’t a valid Zeek type. 

A hook body is allowed to return before it breaks. Hooks may return either a boolean type or void, but aren’t required to return any value.

Events cannot return a value because they are scheduled through the event loop and don’t have a caller to return to.  

Hopefully this short post provides additional clarity to Zeek’s executable types. One final property of these types which I think is worth mentioning is that function, hook, and event bodies can be printed to screen from a script. This makes it possible to observe and debug the contents of each body. They can also be passed as arguments to fmt() and converted to a string type.